A patient in a Positron Emission Tomography scanner. PET (positron emission tomography) scans use radioactive tracers to examine parts of the body, such as the brain. The patient receives an injection of radioactive tracer. The tracer produces gamma rays that are detected by the scanner. Gamma rays: Gamma rays are produced when the tracer emits a positron. A positron is a particle with the mass of an electron but a charge of 1+. The positron is destroyed upon contact with an electron from a nearby atom in the body, and emits two gamma rays in opposite directions. Producing an image: The scanner detects the gamma rays and produces two-dimensional images like slices through the brain. The scanner’s computer then constructs a three-dimensional image based on the scans.